1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device having circuits formed by thin film transistors (hereafter referred to as TFTs). For example, the present invention relates to electro-optical devices, typically liquid crystal display devices, and to electronic equipment in which electro-optical devices are installed as parts. Further, the present invention relates to a method of manufacturing such devices. Note that the term semiconductor device in this specification indicates a category of general devices capable of functioning by utilizing semiconductor characteristics, and the above-mentioned electro-optical devices and electronic equipment are also included in the category of semiconductor devices.
2. Description of the Related Art
Research is widespread into techniques of forming crystalline semiconductor films, and increasing crystallinity, by performing heat treatment, laser annealing, or both heat treatment and laser annealing on an amorphous semiconductor film, formed on an insulating substrate such as glass. Silicon films are often used for the semiconductor films. Note that the term crystalline semiconductor film in this specification refers to a category of semiconductor films in which crystallized regions exist, and that semiconductor films that are crystallized over their entire surface area are also included in the category of crystalline semiconductor films.
The crystalline semiconductor films have an extremely high mobility in comparison with amorphous semiconductor films. Monolithic liquid crystal electro-optical devices (semiconductor devices in which thin film transistors (TFTs) used for a pixel portion and driver circuits are manufactured on one substrate) can be therefore produced if crystalline semiconductor films are utilized, but cannot be realized, for example, by semiconductor devices manufactured by using conventional amorphous semiconductor films.
However, it is impossible to control crystal orientation, so that its arrangement has an arbitrary direction, in crystalline semiconductor films formed by using heat treatment or laser annealing (a technique of crystallizing a semiconductor film by the irradiation of laser light) to crystallize an amorphous semiconductor film deposited by plasma CVD or LPCVD. This becomes a source of limitations in the electrical characteristics of the TFTs.
EBSP (electron backscatter diffraction patterning) exists as a method of analyzing crystal orientation of the surface of a crystalline semiconductor film. The EBSP method can show the crystal orientation directed toward the surface for crystal grains at measurement points in different colors and, can distinctly display focusing upon a certain measurement point, regions within a crystal orientation deviation angle range (permissible deviation angle), set by a user making the measurements, in neighboring points. It is possible for the user to freely set the permissible deviation angle, but the permissible deviation angle is set to 15xc2x0 in this specification. Regions having a crystal orientation within a range that is equal to or less than 15xc2x0 between the point focused upon and its neighboring points are referred to as grains. The reason why the permissible deviation angle is set to 15xc2x0 is because the set value in general is 15xc2x0. Grains are formed from a plurality of crystal grains, but can be seen macroscopically as one crystal grain because the permissible angle for crystal orientation is small.
Further, a method recorded in Japanese Patent Application Laid-open No. Hei 7-183540 can be given as one method of crystallizing an amorphous semiconductor film. A simple explanation is presented here. First, a very small amount of a metal element such as nickel, palladium, or lead is added to an amorphous semiconductor film. Methods such as plasma processing, evaporation, ion injection, sputtering, and solution application can be utilized as the addition method. After the addition, the amorphous semiconductor film is then exposed, for example, to a nitrogen atmosphere at a temperature of 550xc2x0 C. for 4 hours, forming a crystalline semiconductor film. Not only can the electric field effect mobility be increased if a TFT is formed by using such the crystalline semiconductor film, but it is also possible to make the sub-threshold factor (S value) smaller, and to greatly increase the electrical characteristics. The optimal heat treatment temperature and heat treatment time for crystallization is dependent upon the amount of the metal element added and the state of the amorphous semiconductor film. Further, it has been verified that it is possible to increase the crystal orientation property in a monotonic manner by using this method of crystallization.
TFTs have been made smaller in order to provide higher integration and higher speed for present-day LSIs, and TFT size has broken through the 1 xcexcm level. In the case where TFTs of this type are manufactured using crystalline semiconductor films formed by conventional methods of crystallization, if the crystalline semiconductor are patterned for element separation to be separated, then dispersion will develop in active regions of individual devices in that many grain boundaries will exist in some elements and other elements will be formed by almost only grains. Further, if semiconductor films are crystallized using a metal element to promote crystallization, then crystal grains formed having the metal elements as crystal nuclei are mixed with crystal grains formed by spontaneous nucleation (cases in which nucleation begins at a site other than a metal element are defined as spontaneous crystallization within this specification). Dispersion in the semiconductor film properties thus develops. Note that spontaneous nucleation is known to develop at a high temperature greater than or equal to 600xc2x0 C., and when the required crystallization time is long. This dispersion is a cause of dispersion in electrical characteristics and a factor in display irregularities if the crystalline semiconductor films are used as display portions of electronic equipment.
A method of suppressing the grain dispersion in the active regions of individual devices by making the grains smaller is considered here. The crystal nucleus generation density may be increased for this method. Namely, the surface energy of the semiconductor film is reduced, and the critical nucleus radius is reduced by increasing the chemical potential of the semiconductor film. A method of adding to the semiconductor film a large amount of a metal element for promoting crystallization, thus changing the surface energy and the chemical potential of the semiconductor film, is one method of suppressing the grain dispersion. A large number of crystal nuclei are generated by the metal elements if this method is used, and the grains can be made smaller. However, there is a problem with the aforementioned method in that an excessive amount of the metal element remains as a metal compound within high resistance regions (channel forming regions and offset regions). The metal compound allows electric current to flow more easily, reducing the resistance of regions that must be high resistance regions. This becomes a problem that can harm the stability of the TFT electrical characteristics as well as the reliability.
The present invention is a technology for solving problems like those stated above. The present invention is a technique for averaging the number of grains within active regions of individual devices by making the crystalline semiconductor film grains obtained using a metal element smaller without increasing the amount of the metal element used. An object of the present invention is to achieve an increase in the operational characteristics of a semiconductor device, and an increase in its reliability, with respect to the semiconductor device and an electro-optical device, typically an active matrix liquid crystal display device, using TFTs.
The present invention is characterized in that thermal crystallization of a semiconductor film utilizing a metal element is performed after exposing the semiconductor film to a plasma atmosphere. As already discussed, the density of crystal nuclei can be increased, if the critical nucleus radius is made smaller and the surface energy and the chemical potential of the semiconductor film are changed by some type of method. In the present invention, the chemical potential of the semiconductor film is increased, and the density of crystal nuclei generated by the metal element is increased, by performing exposure of the semiconductor film to an atmosphere that has been made into a plasma. If the density of crystal nuclei generated is increased, the amount of time required for crystallization is shortened, and it becomes possible to suppress spontaneous nucleation. The crystalline semiconductor films are embedded into crystal grains that grow with the metal elements as crystal nuclei, and it becomes possible to reduce the grain size with crystalline semiconductor films thus formed. Heat treatment may also be performed after exposing the semiconductor film, to which the metal element has been added, to the plasma atmosphere.
In addition, laser annealing may be performed to improve the crystallinity after exposing the semiconductor film to the atmosphere in which a gas is turned into a plasma and performing thermal crystallization using the metal element, or after exposing the crystalline semiconductor film added with metal element to the plasma atmosphere. It is possible to perform sufficient laser annealing without laser annealing becoming a cause of surface roughness, even if the semiconductor film is exposed to the plasma atmosphere before laser annealing is performed.
According to a structure of the present invention, there is provided a method of manufacturing a semiconductor device comprising the steps of:
exposing a semiconductor film to a plasma of a gas selected from the group consisting of an inert gas, nitrogen, and ammonia;
providing the semiconductor film with a metal containing material; and
crystallizing the semiconductor film by heating after providing the metal containing material.
Films such as amorphous semiconductor films and microcrystalline semiconductor films exist as the semiconductor film in each of the aforementioned structures, and chemical compound semiconductor films having an amorphous structure, such as amorphous silicon films and amorphous silicon germanium films may be applied.
Further, a plasma generation apparatus can be used in order to expose the semiconductor film to the gas plasma atmosphere for the above structures. It is preferable to use a plasma CVD apparatus or a dry etching apparatus as the plasma generation apparatus.
Further, the atmosphere may be an atmosphere having as its main constituent one or a plurality of elements selected from the group consisting of inert gas elements and nitrogen. It is possible to perform sufficient laser annealing even if laser annealing is performed after exposing the semiconductor film to the plasma atmosphere having such elements, for example, without laser annealing becoming a cause of surface roughness. Further, these elements exert no influence on the semiconductor characteristics, even if they exist within the semiconductor film. The semiconductor film may also be exposed to an atmosphere in which ammonia has been made into a plasma.
Further, in the above-mentioned manufacturing processes, the metal element is one element or a plurality of elements selected from the group consisting of Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt, Cu, Ag, Au, Al, In, Sn, Pb, P, As, and Sb.
Further, according to another structure of the present invention, there is provided a method of manufacturing a semiconductor device comprising the steps of:
providing a metal containing material to a semiconductor film;
exposing the semiconductor film to a plasma of a gas selected from the group consisting of an inert gas element, nitrogen, and ammonia, after providing the metal containing material; and
crystallizing the semiconductor film by heating after exposing the semiconductor film.
Films such as amorphous semiconductor films and microcrystalline semiconductor films exist as the semiconductor film in each of the aforementioned structures, and chemical compound semiconductor films having an amorphous structure, such as amorphous silicon films and amorphous silicon germanium films may be applied.
Further, a plasma generation apparatus can be used in order to expose the semiconductor film to the gas plasma atmosphere for the above structures. It is preferable to use a plasma CVD apparatus or a dry etching apparatus as the plasma generation apparatus.
Further, the atmosphere may be an atmosphere having as its main constituent one or a plurality of elements selected from the group consisting of inert gas elements and nitrogen. It is possible to perform sufficient laser annealing even if laser annealing is performed after exposing the semiconductor film to the plasma atmosphere having such elements, for example, without laser annealing becoming a cause of surface roughness. Further, these elements exert no influence on the semiconductor characteristics, even if they exist within the semiconductor film. The semiconductor film may also be exposed to an atmosphere in which ammonia has been made into a plasma.
Further, in the above-mentioned manufacturing processes, the metal element is one element or a plurality of elements selected from the group consisting of Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt, Cu, Ag, Au, Al, In, Sn, Pb, P, As, and Sb.
The density of crystal nuclei generated by the metal elements can be increased, and dispersion in the semiconductor film properties can be reduced by applying the present invention as described above, and therefore semiconductor device performance can be greatly increased. For example, the number of grain boundaries contained in channel forming regions of TFTs can be made uniform. It therefore becomes possible to reduce dispersion in the on current value (the value of the drain current flowing when the TFT is in an on state), the off current value (the value of the drain current flowing when the TFT is in an off state), the threshold value voltage, the S value, and the electric field effect mobility. Further, crystallization becomes possible in a short amount of time, and therefore the amount of processing time can be shortened, and cost reductions can be achieved.
In addition, it becomes possible to make the number of grains in the active regions of individual devices uniform by making the grains smaller. It also becomes possible to reduce dispersion in the electrical characteristics, and reduce display irregularities when using the TFTs as display portions in all types of semiconductor devices.